1. Field of the Invention
The present invention relates to a digital-to-analog converter for converting digital signals to analog signals, and more particularly, to a digital-analog converter for use in an apparatus, such as a video signal-processing apparatus, in which signals must be processed at high speed.
2. Description of the Related Art
Most video signal-processing apparatuses includes a high-speed D/A (digital-analog) current-segment type converter. This type of converter comprises a plurality of constant current sources each having a switch connected thereto. When the D/A converter is designed to convert an n-bit digital signal to an analog signal, it then comprises 2.sup.n -1 current sources and 2.sup.n -1 switches. This is because an n-bit digital signal is changed into 2.sup.n decoded outputs, and the current supply from all constant current sources must be stopped when all bits of the input digital signal are "0".
The 2.sup.n -1 switches are selectively turned on in accordance with the results of the decoding of the digital signal. Therefore, the value of the analog output of the current-segment type D/A converter increases by the output current of one current source when the content of the input digital signal increases by one.
The full scale of the analog output of such a D/A converter, i.e., the output range from the minimum value to the maximum value, is determined by the structure of the device, or the output current values of the current sources. Once the D/A converter has been manufactured, its full-scale value cannot be altered. Consequently, the use of the D/A converter is limited.
Since the output current of each current source changes with variations in the ambient temperature and/or the power-supply voltage, the analog output of this type of D/A converter therefore tends to be unstable during operation. As a result, this D/A converter can hardly achieve a high-accuracy D/A conversion of signals.